This invention relates to balancing apparatus for a magnetic head, and more particularly to such apparatus for providing an acceptable balance over a wide temperature range for a "U" shaped magnetic head with a central pickup member.
Magnetic heads are employed in a variety of circuits for responding to magnetic data on a tape, credit card and in other system applications. A particularly unique and useful apparatus is disclosed in U.S. Pat. No. 4,130,242 entitled Data Storage and Retrieval System Employing Balanced Magnetic Circuits issued on Dec. 19, 1978 to Robert S. Mannion and assigned to the Assignee herein.
In the above cited patent, a data storage retrieval system employs a magnetic circuit read head which essentially consists of first and second parallel pole members. Positioned between the poles is a pickup means which may be mounted on a common base of the pole members to provide an "E" shaped core. The pickup member is centrally located and has a pickup coil associated therewith. The member as positioned between the poles is located at a point which provides a zero output across the pickup coil when the parallel poles are excited by a source of alternating potential. A magnetic material which preferably is positioned on an information carrying card, when inserted in the vicinity of the poles, will effect the flux lines between one of the poles and the pickup means. Based on whether the magnetic material is positioned on the left or the right of the pickup coil, it will provide a signal across the coil indicative of a binary one or a binary zero. Based on the unbalance, the pickup coil provides a sinusoidal signal of a reference phase indicative for example, of a binary one or of a phase shifted 180.degree. with respect to the reference phase; which phase may be indicative of a binary zero.
In any event, as described in the above noted patent, such magnetic heads are balanced to provide a null voltage relatively equal to zero from the pickup coil during a quiescent condition. Essentially, if this system is operated in a controlled environment such as in a bank, a store, or in an indoor environment maintained at a relatively constant temperature, there is no problem in maintaining a null or a balance based on temperature of operation. If the unit is not operated in a controlled environment as, for example, outdoors or in any environment where there is a substantial change in temperature, there is a problem manifesting itself in a change in the null. As can be ascertained from the above noted patent, a proper null is important to proper operation. Based on temperature changes, the respective coils on the magnetic core shift in a random fashion and hence, produce nulls which are unpredictable as being a function of the particular shift of a particular coil and hence, the shifting of the null voltage occurs in a relatively random fashion in response to temperature changes.
To achieve a good null, the magnetic head must exhibit almost perfect quadrature. Essentially, the resulting magnetic flux from the parallel poles must be equal in both phase and amplitude. Due to the fact that the changes in null are random and non-repeatable for each coil, prior art feedback techniques are not applicable.
Conventional compensating techniques require a plurality of complex and sophisticated circuitry which, essentially, increase the cost and complicate the system operation. The difficulty resides in that to achieve quadrature, one must implement two separate feedback loops and hence, one loop is necessary to control amplitude, while a second loop is necessary to control phase. The control of amplitude and phase must be implemented by means of reliable and economical circuitry, while maintaining a minimum of circuit components to achieve a null voltage over a wide range of temperatures.